The Basics of
Evolution

The Fundamental Process

Biological
evolution occurs when individual organisms with particular characteristics
replace other individuals with other characteristics. It is not that the characteristics of any single individual
change, or evolve, during its lifetime. Rather, each individual lives its life in its own way, producing
offspring if and when it can.

The
key to evolution is the degree of success with which each individual produces
offspring. Any
genetically-heritable characteristics can be passed on to offspring. Consequently, those individuals that
have the most offspring contribute the most to future generations. Those individuals that have the least
offspring contribute the least to future generations. Over the course of many generations, some
genetically-heritable characteristics are lost from the population overall,
while others become common.

Evolution
can occur only if there are different genetically-heritable characteristics in
a population of organisms. There
must be genetic diversity. In
nearly all populations of nearly all species, there is genetic diversity. Partly, this is because the genetic information is
re-assorted at each generation. It
is also because mutations occasionally occur, causing changes in genetic
information. Once a mutation
occurs, and is passed on to the offspring of the first individual to carry that
mutation, then the mutation becomes a part of the genetic diversity of the
population.

Evolution
can occur rapidly or very slowly. It occurs slowly, if at all, when the particular characteristics of a
population make it well-suited to the environment in which it lives. Under such conditions, most mutational
changes make individuals less suited to
the environment. As a result, the
more common characteristics remain the norm.

Rapid
evolution can occur during times of environmental change. A population's environment can change
if the individuals in that population migrate to a new location. Or, environmental conditions can change
for numerous reasons, including long-term climate change, the introduction of a
new species, or the loss of a previously-common species. In a new environment, genetic
variations that were previously uncommon may be advantageous. Individuals with these variations may
now out-compete their fellows, and their genetic variations may become the new
norm for that population.

The
fundamental principles of evolution are these:

1. Evolution depends upon genetic variation.

2. Evolution occurs as some genetic variations become common in
populations, and others become rare.

3. The source of genetic variation is mutation. Genetic variation can be augmented by
genetic reassortment during the production of offspring.

4. Individual organisms do not change or evolve. Evolution is the replacement, over the
course of numerous generations, of some genetic variations by other genetic variations.

5. It is not possible to mutate in anticipation of environmental
change, or to direct mutations to specific characteristics. Mutation is not a conscious process.

Below, we explore some of the details:

DNA, Genes, Mutations, and the Characteristics of
Organisms

Genetic
inheritance depends upon genes, which are segments of DNA, the fundamental
chemical of chromosomes. DNA
carries the "information" that determines how organisms grow and
develop, and that determines many of their characteristics. It does not dictate all of an individual's characteristics, because many
aspects of most species are shaped by the peculiarities of the environment in
which they live.

Every
individual of every species begins life as a single cell. In the case of humans, that single cell
is the fertilized egg, which contains one set of chromosomes contributed by the
mother, and one set of chromosomes contributed by the father. As the fertilized egg divides and the
cells differentiate to become all of the different cell types of a human, the
DNA molecules of the fertilized egg must be duplicated over and over, so that
each of our trillions of cells contains an exact copy of the DNA contained in
the fertilized egg. DNA
replication must be tremendously accurate to ensure that every cell contains the
information that it needs.

Although
DNA replication is tremendously accurate, it is not 100% accurate. Occasionally, mistakes are made. If mistakes occur in the DNA of genes,
then those genes are altered.

DNA is a chemical. Therefore, it
follows the laws of chemistry. Consequently, DNA molecules can be damaged--chemically altered--by
radiation, chemicals, cosmic rays, oxygen radicals, etc. Although DNA damage can often be repaired, it is not always repaired,
and repair may be imperfect. Damage and/or imperfect repair can also alter genes.

These
changes to DNA are mutations. Because they occur by normal, chemical mechanisms, it is impossible to
prevent them from occurring. It is
also impossible to cause them to occur in specific genes . They
occur at random.

"Random mutation" does not mean "un-caused mutation." It means that the mechanisms that cause mutations cannot choose which part of a DNA molecule to affect. This is illustrated in the figure on the right, which shows an oxygen radical (a common mutagen) inside the nucleus of a cell. It is surrounded by DNA, from many different genes, but all of the DNA is chemically the same. The oxygen radical has an equal likelihood of reacting with any nucleotide in any DNA molecule near it. The probability that a base will be modified, and thus cause a mutation, is statistically random.

It
is tremendously important to recognize that mutations are changes in DNA. A
person cannot mutate. A turtle
cannot mutate (and turn into a ninja). Why not? Because a chemical
mistake in the DNA of one cell affects only that cell, and is not spread
throughout the body to all of the cells. A mutation in the DNA of an adult human will not change that
person [unless the mutation occurs in a gene that controls cell division, in
which case the mutated cell may begin to divide uncontrollably, and become a
cancer].

To
change the characteristics of a whole organism, a mutation must occur in a cell
in the gonads, destined to become an egg or sperm, and become incorporated into
a fertilized egg, and develop into a complete individual. Only then can a new mutation, a new DNA
change, become a part of every cell in an individual organism. Only then can a new mutation change the
characteristics of the organism. In other words, if an individual is exposed to mutation-causing
chemicals or radiation, that individual will not mutate . However, that individual's offspring may carry mutations. Once the offspring reproduce, and pass DNA changes to the
next generation, then the mutations become part of the genetic diversity of
that species.

If that mutation gives an individual an advantage, so that the individual is more likely to produce healthy offspring, then the numbers of individuals with that particular genetic variation will increase with each generation that passes. If a mutation gives an individual a disadvantage, so that the individual reproduces less successfully (or dies), then that particular genetic variation will be lost from the gene pool of that species.

Genes, Proteins, and Cellular Micromachines

To
understand how mutations can change the characteristics of organisms, it is
necessary to understand how genes work. In general , genes are segments of
DNA that carry the information for proteins. Genes do no more than this. Inside cells, genes just sit there, waiting for their
information to be used. "Using" the information means following the chemical processes
by which cells produce proteins.

We
will ignore, for now, the process that cells use to produce proteins. It is enough to say that the information
in each gene dictates the production of a single type of protein. Human DNA is estimated to contain
information for around 30,000 different proteins. Each of these proteins is a specific kind of cellular
"micromachine" that has a specific function.

Proteins
are produced by assembling "building blocks" called amino acids. There are 20 different amino acids that
are used in cells. From these, an
infinite number of different proteins can be built, depending on how many of
these building blocks are strung together, and the order in which they are
assembled. The differences in
functions of proteins depend on the differences in amino acid sequence of the
proteins.

A
mutation in a gene--a change in the DNA--has the likely consequence of changing
the amino acid sequence of the protein whose information that gene
carries. This, in turn, can change
the way that the protein micromachine works. To see the types of effects that this can have on the
characteristics of an entire organism, it may be best to discuss some specific
examples:

1. Eye color

Eye
color in humans is determined by several different genes that produce different
kinds of proteins. One gene,
called EYCL3 , carries the code for an
enzyme (a protein that catalyzes a chemical reaction) that produces a brown
pigment. This gene is used, or
"turned on" in the cells of the iris. An individual who inherits a functional gene for this enzyme from either parent will be able to
produce the brown pigment in her irises, and will have brown eyes. Mutations in this gene can cause the
protein not to work. An individual
who inherits non-functional genes for this enzyme from both parents will be unable to produce the brown
pigment. As a result, the
individual will have green or blue eyes. (The green pigment also depends on a gene that produces an enzyme; the
blue color is a result of the way that iris cells reflect light, and is not
based upon a blue pigment).

This
particular gene need not be either functional or non-functional. As with any gene, there are many, many
different variations possible. One
variation may produce a protein that is a very active enzyme; individuals with
this version of the gene will have very dark brown eyes. Another variation produces an enzyme
that works, but not very well. This enzyme cannot produce as much brown pigment. Individuals with this version of the
gene will have light brown eyes.

The gene is the set of instructions for the enzyme; the enzyme produces the pigment. Variations in the gene sequence, resulting from mutations, create enzymes with varying degrees of activity. This shows up in the human population as variation in the intensity of brown color in the irises of our eyes.

2. Hair color
and skin color

The
story for hair color and skin color is similar to that for eye color. The genes that determine the color
carry the information for enzymes that produce pigments. If we produce the enzyme, we make the
pigment (brown hair, or brown skin). If we do not produce the enzyme, we do not make the pigment (blonde
hair, or light skin). Different
variations of the genes result in different variations in the individual's
characteristics.

3. Alcohol
tolerance

Some
individuals cannot tolerate alcohol. This is the result of carrying a particular version of the gene, ALDH2 . It is
thought that the version of the gene that produces this characteristic first
arose in Asia, since the inability to tolerate alcohol is most common in Asian
populations.

Alcohol
is metabolized by two enzymes. The
first (ADH) converts alcohol to acetaldehyde. The second (ALDH) converts acetaldehyde to acetic acid. We can then use the acetic acid in our
energy-metabolism pathways.

Although
alcohol itself interacts with our brain cells to make us feel giddy (among
other things), acetaldehyde is toxic, and makes us feel sick (or worse). Therefore, the ability to tolerate
alcohol depends, in part, on how rapidly we can convert acetaldehyde to acetic
acid. The version of the ALDH2 gene that results in inability to tolerate alcohol
produces a protein that prevents acetaldehyde conversion. Individuals with this genetic variation
metabolize alcohol to acetaldehyde, but are unable to get rid of the
acetaldehyde. As a result, they
become sick very quickly, and rapidly learn to avoid alcohol. Although
this may sound unfortunate, it turns out that this particular genetic characteristic
provides virtually 100% protection against becoming an alcoholic.

Genes and Morphology

The several genes and proteins described above provide some examples of how mutations in DNA can cause changes in the characteristics of individuals. In each case, however, we have discussed enzymes that affect pigments or metabolism, and not genes that affect morphology--the shapes of organisms. Most people, when they think of evolution, envision images of animals that look very different from each other, such as fish and mammals, or dinosaurs and birds. These differences are very dramatic, and may seem unlikely to occur by mechanisms similar to those that can influence characteristics as simple as hair color. And yet, the same rules apply.

For
any organism, or organ within that organism, the component parts are in a
specific, characteristic relationship to each other. This applies even at the level of single cells. Skin cells are on the outside,
intestinal cells on the inside. How do these cells "know" what to do?

Every
cell type adopts its particular "identity" by "turning on"
a specific set of genes. Of the
30,000 or so genes in human chromosomes, each cell type expresses only a
subset. It is the combination of
genes that are used in a cell that gives it its particular
characteristics. This means that
cells don't "know" what to do; rather, the set of genes that they
express makes them what they are. But, this just pushes the Difficult Question one step farther: what
determines which genes are expressed in any particular cell?

There is more to a gene than just the DNA segment that provides the information for a protein. There are also DNA segments that provide information for when and where that gene should be used to produce protein. These DNA segments that regulate the use of genes are called, not surprisingly, regulatorysequences, or sometimes, control elements. Every gene has one or more control elements; some genes have many control elements, allowing them to have very complex patterns of regulation.

Control
elements, by themselves, do little. To function, they must have specific proteins bound to them. We might think of control elements as a
kind of "parking spaces" on the DNA, where different types of
proteins can park. When one of
these specific types of proteins has parked on the DNA, it signals to the
cellular machinery that the nearby gene is to be used. Thus, what makes a cell become a
particular type of cell is the gene-control proteins that it contains. These proteins control which genes are
used in that cell.

Great. Now we know the mechanism by which cells produce different proteins. But, it doesn't answer the Difficult Question yet. Again, it pushes it one step further: what determines the production of these gene-control proteins?

To
answer this question, we need to discuss some embryology. A fertilized egg is not exactly
homogeneous. Rather, it contains a
variety of chemicals, some of which are proteins, whose amounts vary within the
egg. This is illustrated in the figure
below, in which each color represents a different chemical. This asymmetric distribution of chemicals enables the egg to
have a definite "top" and "bottom," "head" and
"tail," and "left" and "right," even before any obvious
embryological development has occurred.

When the fertilized egg starts to develop, it first divides into two smaller cells, then these divide to form four even-smaller cells, then these divide to form 8 cells, and so on. For a "generic" vertebrate embryo, we see something like that shown in the figure below. On the right-hand side of the top row, we see the stage called the blastula, which is a hollow ball of very small, "normal-sized" cells. In cross section, colored to match the figure above, this hollow ball would look somewhat as shown on the right. Cells in different regions of the embryo would have different amounts of the different proteins or other chemicals that were distributed asymmetrically in the original fertilized egg. Cells on the top contain different molecules than cells on the bottom.

It appears to be sufficient to specify head vs tail, and dorsal (back) vs ventral (belly); this automatically establishes left and right.

The
molecules we have illustrated here in different colors control gene
expression. They can be either
gene-control proteins, or chemicals that activate gene-control proteins. As a result of the uneven distribution
of these proteins, each cell in the embryo acquires its own combination of
these proteins, in specific concentrations. The result is that cells in different parts of the embryo
"turn on," or begin to use, different genes.

As
cells begin to use different genes, they begin to do different things. Among the things they do is move. The first set of movements, called
gastrulation, brings cells from different parts of the embryo into
contact. The contact between cells
acts as a signal to activate new sets of genes, which causes new cell
movements. Through the repeated
sequence of cell movements, cell contact, and gene activation, the business of
embryo development is controlled--and the numbers of different kinds of cells
increases.

The morphology of the embryo, and of the
animal that finally is formed, is established by this series of cell movements
and changes in gene expression. The overall controls are the proteins that are responsible for
communication between cells, the gene-control proteins, and the cellular
information-relay system that enables cell-cell communication to change the patterns
of gene activation. This basic
description applies to all of the developing portions of the embryo.

An
important feature of this kind of embryo developmental control system is that
it works on a very small scale. When the embryo is only 2 cells, or 4 cells, or 8 cells, then the entire
embryo can communicate. As the
embryo grows larger, however, and as the number of cells increases, then the
control systems function only on small portions of the embryo. As the eye begins to form, these kinds
of gene-control systems operate within the "field" of eye-forming
cells. As the ear begins to form,
these kinds of gene-control systems operate within the "field" of
ear-forming cells. As the liver
begins to form, or as the pancreas begins to form, there are similar kinds of
controls.

One
organ-development system for which a great deal is known (but not yet
everything) is the limb. We will
discuss it as an example, recognizing that other organ systems undoubtedly
develop through similar kinds of genetic control mechanisms.

Vertebrate Limb Development--an Example of the
Development of Body Parts

At a relatively early stage in development (in the
figure above, the last image) vertebrate embryos develop a series of bumps
along their backs, and two small nubbins on each side. The two nubbins are the limb buds. There are two limb buds on the right
(forelimb and hindlimb), and the corresponding two on the left. These begin to form when the embryo
reaches a developmental stage at which each body segment begins to
differentiate from the others.

The
limb buds are very small -- small enough that cell contact and
diffusible molecules can serve as cell-cell communication mechanisms to
establish cellular identities. A
variety of genes are activated in limb buds. Some produce diffusible short-range hormones, some produce
gene-control proteins. In early
stages of limb bud development, an abbreviated pattern of gene expression is something like that shown in the diagram
shown above (see Tickle, 2000, www.ijdb.ehu.es/fullaccess/fulltext.feb00/Tickle.pdf). These different genes establish
cellular identities in the limb bud, and consequently establish the
"pattern" of limb development.

If
we use a human arm to provide terminology, we can describe it this way: The expression of Shh (red in the
diagram), determines the "thumb-to-pinkie" pattern. The pinkie forms on the side closest to
the cells that activate the Shh gene. The different Hox genes ensure that the digit next to the pinkie is the
4th finger, and the one next to that is the 3rd finger,
and the one next to that is the index finger, and the farthest is the
thumb.

As the limb grows outward
from the flank of the embryo, additional genes and diffusible molecules (such
as retinoic acid, a form of Vitamin A) establish the positions of shoulder,
upper arm, forearm, wrist, and fingers. This same basic pattern has been seen in the developing limbs of the
vertebrates that have been studied.

Because these developmental events all occur in a very small piece of tissue, encompassing a relatively small number of cells, any changes in the cell-cell communication systems or the diffusible molecules can have dramatic effects on the overall pattern that eventually forms. A chicken wing differs from a human hand only in being smaller overall, having the outer digits fail to develop, and having the cells between the digits fail to die. All of this is controlled by the relative positions of cells that activate the genes for certain gene-control proteins, under the influence of diffusible small molecules, and cell-cell contact.

As
we said above, we can use the limb as an example of the kinds of controls that
govern the development of body parts. Whether limbs, eyes, livers, on pancreases, all body parts start out as
very small groups of cells in which cell-cell contact and diffusible molecules
can set up a pattern of which cells activate which gene-control proteins. As the gene-control proteins activate
additional genes, the developing organ acquires the various protein
micromachines that build the structures.

Internal Controls in Developing Systems

An
important discovery is that biological systems are capable of regulating their
development to ensure that all of the parts are in the correct relative
positions. An excellent example is
seen in an experiment performed by Barry Sinervo (Sinervo, B. and Huey, R.B. 1990.
Allometric engineering: testing the causes of interpopulation differences in
performance. Science 248:1106-1109.): he removed material from one lizard egg
and injected it into another lizard egg, thereby producing one egg that was
much smaller than normal, and one that was much larger than normal. The lizards that hatched from these
eggs were normally-formed lizards, but smaller or larger than normal. This kind of experiment (manipulating
eggs or embryos) has been performed with many different species, and leads to
the very clear conclusion that developing systems can control their relative
proportions. That is, removing
half of the material in an egg does not result in a lizard with half of its
body parts. It produces a
normal-looking, but small, lizard.

A
more recent finding shows the importance of this kind of "internal
control," as illustrated in
the figure below. A genetic change
that alters the length of a fish jaw does not just produce fish with jaws that are too short in an otherwise normal
head. Instead, the
entire head changes shape to accommodate
the shorter jaw. This is somewhat
surprising, since the jaw and the other structural elements of the head develop
from different groups of cells. Apparently, the changes in the head result from the same kind of
developmental plasticity as was seen with the too-large and too-small lizards.

This
is important because it illustrates that a mutation that alters one piece of a
very complex structure need not cause the structure to fail, because developmental
controls can often accommodate the change .

There
is a general misconception, or perhaps "worry," that a mutation that
changes the size of one body part relative to others would disrupt the entire
organism. The pieces wouldn't fit. We now know from experimental
manipulations and from observations of naturally-occurring mutations, that the
pieces do fit. Developmental controls don't just make the pieces; they make
the pieces and the connections
among the pieces, so that the overall organism works.

The Evolution of Morphology

As
we said above, evolution occurs when individuals carrying some particular
genetic variation have more offspring than others, and over the course of many
generations, out-compete their fellows. We also said that the source of genetic variation is mutation--changes
in DNA. The three sections,
"Genes and Morphology," "Limb Development," and
"Internal Controls in Developing Systems," provide a brief summary of
how genes can control the shapes of organs, limbs, and the whole organism. By linking these different concepts, we
can understand how evolution of morphology can occur.

A
genetic mutation--a change in DNA--can alter the function of a protein if it occurs within the part of the
gene that is the code for the protein. A mutation can also change the time or place that the gene is activated, if it occurs in a gene's
control elements. Any of these
kinds of mutations can alter the morphology (shape) of an organism. The internal controls of development
ensure that an altered body part still integrates properly with the rest of the
organism.

This
means that evolution of morphology can occur through the mutation of single
genes. It is not necessary for all
of the genes to mutate at once.

Thus,
it requires only minor alterations in the developmental controls in early limb
development to cause differences in overall limb morphology that we see as seemingly quite dramatic, such as those
illustrated below.

These
considerations of the limbs of existing vertebrates provides us with insights
into the evolution of limbs. Fossil evidence suggests that land animals are the descendents of
lobe-finned fish similar to the coelacanth. Unlike most fish today, these fish (both the living ones and
the fossilized ones) display bones in their fins that are remarkably similar to
the bones in modern animal limbs. We can understand, based on the molecular and developmental biology of
limb development, how mutations in the genes that control fin (limb)
development could, over the course of millions of years, result in limbs such
as we would find on an amphibian.

As
described by Carl Zimmer in At the Water's Edge--how life came ashore and
then went back to sea again , fossils of
animals from Greenland give us insights into this transition. Plant fossils
associated with the fossils of these animals indicate that they lived in swampy
environments, somewhat similar to Mangrove swamps today. In this environment, large fish would
have difficulty swimming among the tangled vegetation. However, pushing with their fins would
be very effective. Pushing
provides the selective pressure, which would enable the occasional individual
with slightly stronger fins to catch prey more easily, and escape predators
more readily. Over the course of
numerous generations, stronger fins would become the norm in the
population.

This
example of limb evolution, under-pinned by an understanding of limb
development, provides insight into several principles of evolution.

- Evolution results from the increase in
frequency of particular genetic variants in a population, at the expense of
other genetic variants.

- Genetic variation results from mutation
of DNA.

- Mutations in DNA affect the activity or
expression pattern of proteins--micromachines that carry out the mechanics of
life.

- Evolution usually occurs by
modification of pre-existing structures, rather than the appearance of
altogether new structures. In the
case of limb evolution, the pre-existing structures were the fins of
lobe-finned fish.

- Changes to one part of a complex
structure, resulting from mutation, can often be accommodated by compensatory
changes in the rest of the complex structure, not by additional mutation, but
through internal control mechanisms that operate during embryo development.

Natural
Selection – If Mutation is Random, Why Does Evolution Occur at All?

Every
time that scientists have examined the process of mutation, seeking to learn if
there are recognizable patterns, the answer seems to be that mutation is
essentially random. If we think of
the causes of mutation, such as chemical mistakes in DNA replication or repair,
or physical damage due to cosmic rays or other radiation, we see that there is
no reason to expect mutation to be anything except random. And yet, evolution has produced
highly-complex life forms, with a great many specialized adaptations that make
them appear as if they were specifically designed to live where and how they
do. How can an apparently random
process result in apparently directed evolution?

This
question is one of the "logical" problems that many people have with
evolution. It is simply counter-intuitive that a random process can give rise
to highly-ordered structures, and to adaptation to specific environments. Perhaps the best way to address this
issue is to provide some examples in which we model the process.

Example 1: using colors to represent individuals

In
this example, we consider two populations of 10 individuals. Each individual can reproduce, but
because of ecological constraints (food supply, for example), the environment
maintains the population at a maximum of 10 individuals. Using random choice (throwing dice, for
example), we have assigned different colors to the 10 individuals--and we use
the same colors for each population. The two populations look like this:

The left-hand population is in a cool environment, then
individuals colored blue, green, and purple have a competitive advantage over
the other colors, with purple being most successful. The right-hand population is in a warm environment, with
red, orange, and yellow having a competitive advantage. Red is most successful. In the next generation, we have this:

There isn't really much difference between this generation and
the previous generation. Most of
the different colors (genetic variants) are present in each population. Some of the variants have increased in
frequency, while others have decreased in frequency. Compared to the prior generation, each one looks pretty much like the parental generation, except that some of the genetic variations are a bit more common.

In the next generation, we have this:

And then this:

And then this:

Each population changes slowly with time, from generation to
generation, as some individuals have more offspring than others. The two populations began with
identical genetic diversity, based on random "mutation." However, the environmental conditions
were different, so selection was
different. The cool environment
selected for the cool colors, and against the warm colors. The warm environment selected for the warm colors, and against the cool colors. Mutation was random, but selection provided a direction to
the evolution.

Example 2: leaf shape

In
this example, a species of shrub has spread across a valley, and up into the
mountains on either side of the valley. As the climate warms, the shrubs lower down (in the valley) die out, and
the shrubs higher up (in the mountains) survive. But, on the north side of the valley, the shrubs are on
mountain slopes that receive full sun; rain water dries rapidly. On the south side of the valley, the
shrubs are on mountain slopes that receive little sun; rain water dries
slowly. Thus, one population is in
a dry environment, and the other is in a wet environment.

Both
populations start with identical genetic diversity, resulting from random
mutations that affect leaf shape. Some leaves are wide, some are narrow, some are in-between. They look like this:

Wet environmentDry
Environment

Of course, the shape of a leaf is not a trivial matter, if
you are a plant. A broad leaf can
capture more sunlight, perform more photosynthesis, and thus provide more food
for the whole plant. A narrow leaf
is much less effective. However,
the more broad a leaf is, the more stomata it has--openings that allow CO2 to enter and O2 to escape. The more stomata a leaf has, the more water it loses by
evaporation. Therefore, a plant
with broad leaves requires more water than a plant with narrow leaves; it is
much more likely to wilt on a hot, dry day. From these considerations, we can see that in a wet
environment, where water loss is not a serious problem, wide leaves would be
advantageous. However, in a dry
environment, where water loss is a
problem, wide leaves would be a liability.

After a few generations, the distributions of leaves in the
two populations look like this:

After a few more generations, they look like this:

And, after a few more generations, they look like this:

Wider leaves lose more water during the day, so in the dry
environment, wide leaves are selected against. Narrow-leaved plants
produce more seeds than wide-leaved plants. However, wide leaves can carry out more photosynthesis than
narrow leaves can, so in the wet environment, wide leaves are selected for . Wide-leaved plants produce more seeds than narrow-leaved plants.

The
genetic variation in leaf shape was determined by random mutation. However, the environmental conditions
determined which variations were more successful, and provided a kind of
direction to evolution.

From
these examples, it should be evident that the random nature of mutation does
not cause evolution to be random. Random mutation simply provides an array of genetic variants for
selection to choose among. If there are genetic variants that are successful in
the particular environment, then those genetic variants prosper. They produce more offspring. Eventually, they become the norm.

What
if there are no genetic variants that are particularly successful? What if the environment changes rapidly,
and there just don't happen to be any mutations in the population that enable
any individuals to do well? Then,
as has happened over and over during the history of life on earth, the
population will die out. The
species may go extinct.

Mutating in Order to Survive

We often develop the idea that evolution occurs because a species of plants or animals mutates "in order to survive." It seems as if this must have happened, because the plants and animals that are now alive are well adapted to their environments, and appear to be much better adapted than their ancestors that are recorded in the fossil record. Indeed, the ancestral species died out, which seems to indicate that they were not "good enough" to make it, while the ones that could make themselves "better" are with us today.

This notion is also satisfying from a human-centered view of the world. We are here now, so we must be the "best" species ever to evolve. Ancient species were much more primitive, so evolution would seem to work as some kind of drive to become better -- and eventually become human.

But does this make sense? It is even possible?

If we think about the evolution of plants, in which they acquire narrow leaves in dry climates, or broad leaves in wet climates, we must wonder how they can do this. As far as we know, plants can't think. As far as we know, they cannot predict what the world will be like in a few hundred generations. And, as we know for certain (see above), mutations occur randomly. There is no way to prevent mutations from happening, and no way to cause them to occur in specific genes.

If it is not possible to make mutations happen on cue, it simply cannot possible to mutate in order to adapt to new conditions.

And yet, so many species seem so well-suited to their environments. How can this have happened if they didn't know what they were doing? The examples above should help resolve this conundrum. Random mutation is sufficient to account for the observation, if there is enough genetic diversity in the population that some individuals have characteristics that are advantageous, and enable them to survive changing environmental conditions. As long as environmental conditions don't change too fast, the continued occurrence of mutations is capable of producing genetic variations -- some of which may happen to be useful. Fortunately, the rate of mutation is high enough that useful mutations have occurred sufficiently often to accommodate many of the environmental changes that have occurred.

But not always. If useful mutations don't happen to occur, or if the environment changes too fast (as might occur if a giant asteroid strikes the earth), then species die out. They become extinct. It is instructive to consider the fact that most of the earth's species have gone extinct. Those of us alive now are the lucky remnants of various genetic lineages in which advantageous mutations did occur.

An
excellent example of how selection operates on individuals of a population, and how the individuals do not
"plan" their mutations is offered by the following story, which is
taken from Shadows of Forgotten Ancestors by Sagan and Druyan.

Instincts are inborn behaviors that animals display without
necessarily thinking logically about it. We recognize a great many instincts in different breeds of dogs--such as
chasing and retrieving sticks (retrievers), digging (beagles, selected for
hunting rabbits), and pulling (huskies, selected to work in dogsled
teams). We don't usually think of our
own instincts, but we have them.

One
human instinct that is variable, but present in a significant fraction of the
population is fear of the dark. Many of us recall being afraid of the dark, or of "monsters"
when we were children. Most of us
recall our parents telling us "there's nothing to be afraid of; go back to
sleep." If Mom and Dad were
telling us not to be afraid, then where did we get our fear of the dark? It cannot be a learned behavior, but must be an
instinct.

There
are good reasons to have an instinctive fear of the dark. In our history, before civilization,
the world was a scary place. There
were many predators that hunted at night. In a very real sense, there were monsters out there. The world in
which our ancestors lived was perilous.

So,
picture the following: Our ancestors are sitting around the fire at night--or
maybe they are perched on the limbs of trees.* Most of them have a vague unease, and are afraid to venture
out into the night. But, this
genetically-coded uneasiness, like any genetically-coded trait, is
variable. Some individuals are
more afraid, some are less afraid.

So
here is everyone sitting around being afraid, and one guy says, "You guys
are wimps. I'm going for a
walk." He goes out into the
night, and is eaten by lions.

Whose
genes did we inherit?

This
simple scenario offers several important insights into the nature of evolution
and natural selection. They are:

1. Selection operates on individuals. Individuals can be eaten, or they can
live until morning. The entire
tribe did not get eaten at once, nor did the entire tribe live until morning.

2. "Survival of the fittest"
really is a poor phrase to describe this. We don't think of "the wimps" as being
"fit," but they are the ones who passed on their genes. In evolutionary terms,
"fitness" refers to the production of offspring, and not to anything
else. In this case, being fearful
increased an individual's fitness in this particular lion-filled environment.

3. Selection operates on traits that
already exist. The individuals
who were afraid survived; the individuals who were not afraid were likely to be
eaten. No one was eaten by lions,
and then developed the fear-of-the-dark trait.

4. There is no planning in evolution, no
"mutating in order to survive." Our ancestors did not sit around the fire thinking, "I
bet that in a few hundred thousand years, our descendents will be better off if
they are afraid of the dark--so I'm going to mutate, and become afraid." Instead, each individual lived his or
her life normally. The ones who
happened to have genetically-coded traits that were advantageous in that
environment passed on their genes to more offspring.